This invention relates to the art of papermaking and more particularly to impulse drying of paper webs or sheets. With impulse drying, a web of improved properties can be produced, such as densification, high smoothness and increased strength. Impulse drying, although known, has not yet been commercially practical because of the problem of delamination.
After a paper or paperboard web is formed, as by a Fourdrinier apparatus, it is passed through a number of roll pairs, the rolls usually unheated, to remove water by mechanical pressing and is then contacted by a heated roll to remove water by evaporation. A known treatment to increase strength is to pass a paper web, typically of indefinite length, through one or more heated roll pairs, the rolls typically compressing the web to 70%-80% of its original thickness. Only one roll of the roll pair is usually heated. The heated roll is of a temperature approximately 200.degree. C. An endless porous felt is usually located in the nip and passes around the unheated roll. The combination of heat and pressure exerted on the web by the nip of the rolls works or functions to increase web strength. However, it has been observed that such treatment, termed impulse drying, has the undesirable effect of delaminating the web. This is due to the fact that the web becomes very hot upon contact with the heated roll in the nip of the roll pair. While the web is under pressure in the nip, heat energy from the heated roll raises the temperature of the water within the web to a significantly high value, such high temperature water termed superheated water. Upon exiting from the nip, the roll pressure on the web is suddenly released, with the consequence that the moisture within the web has a tendency to flash and cause delamination of the paper web. Further, the relatively high roll temperature causes significant densification of the web surfaces which is in contact with the heated roll, thus creating a barrier to the escape of the vaporized, superheated water within the web.
Sheet or web delamination has been a major obstacle to the commercialization of impulse drying technology.